Histones are positively charged proteins. DNA is negatively charged because of its phosphate groups, so DNA is attracted to histones and wraps around them. How tightly it is held depends on the chemical groups attached to the histones.Acetylation is the addition of an acetyl group (COCH3) to a histone. The acetyl group, donated by a molecule called acetylcoenzyme A, reduces the positive charge on the histone. The histone then binds the DNA less tightly, the chromatin becomes less condensed, and transcription factors and RNA polymerase can reach the DNA, so the gene can be transcribed.Decreased acetylation of the associated histones inhibits transcription. Removing acetyl groups is called deacetylation, and it is catalysed by enzymes called histone deacetylases (HDAC). Deacetylation increases the positive charge on the histones, so they bind the DNA more tightly. The chromatin becomes more condensed and the gene is switched off.
more acetylation → less condensed chromatin → transcription
less acetylation → more condensed chromatin → no transcription The two epigenetic mechanisms involve changes in opposite directions but have the same effect: increased methylation of DNA inhibits transcription, and decreased acetylation of histones also inhibits it. Exam questions often test this, so read carefully which change is described.Like methylation, histone acetylation can be changed by the environment and is passed on when cells divide. Drugs that inhibit HDAC keep histones acetylated, so the target genes are more likely to be transcribed.